GeoArabia, Vol. 7, No. 1, 2002Gulf PetroLink, Bahrain

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A review of the Pan-African evolution of the Arabian Shield

Pierre Nehlig, Antonin Genna and Fawzia Asfirane, BRGM, Francewith the collaboration of N. Dubreuil, C. Guerrot, J.M. Eberlé, H.M. Kluyver,

J.L. Lasserre, E. Le Goff, N. Nicol and N. Salpeteur, BRGM, France,M. Shanti, BRGM, Jiddah, D. Thiéblemont and C. Truffert, BRGM, France

ABSTRACT

Recent fieldwork and the synthesis and reappraisal of aeromagnetic, geologic, structural,geochemical, and geochronologic data have provided a new perspective on the structuralevolution and geologic history of the Arabian Shield. Although Paleoproterozoic rocksare present in the eastern part of the Shield, its geologic evolution was mainly concentratedin the period from 900 to 550 Ma during which the formation, amalgamation, and finalPan-African cratonization of several tectonostratigraphic terranes took place. The terranesare separated by major NW-trending faults and by N-, NW- and NE-oriented suture zoneslined by serpentinized ultramafic rocks (ophiolites). Terrane analysis using thelithostratigraphy and geochronology of suture zones, fault zones, overlapping basins,and stitching plutons, has helped to constrain the geologic history of the Arabian Shield.Ophiolites and volcanic-arcs have been dated at between 900 and 680 Ma, with thesouthern terrane of Asir being older than the Midyan terrane in the north and the ArRayn terrane in the east.

Final cratonization of the terranes between 680 and 610 Ma induced a network ofanastomosing, strike-slip faults consisting of the N-trending Nabitah belt, the major NW-striking left-lateral transpressive faults (early Najd faults), lined by gneiss domes andassociated with sedimentary basins, and N- to NE-trending right-lateral transpressivefaults. Following the Pan-African cratonization, widespread alkaline granitization wascontemporaneous with the deposition of the Jibalah volcanic and sedimentary rocks intranstensional pull-apart basins. Crustal thinning was governed by the Najd fault systemof left-lateral transform faults that controlled the formation of the Jibalah basins and wassynchronous with the emplacement of major E- to NW-trending dike swarms throughoutthe Arabian Shield. The extensional episode ended with a marine transgression in whichcarbonates were deposited in the Jibalah basins. Continuation of the thinning processmay explain the subsequent deposition of the marine formations of the lower Paleozoiccover.

Our interpretation of the distribution and chronology of orogenic zones does notcorrespond entirely to those proposed in earlier studies. In particular, the N-trendingNabitah and NW-trending Najd fault zones are shown to be part of the same history ofoblique transpressional accretion rather than being two distinct events related to accretionand dispersion of the terranes.

INTRODUCTION

The general theory of plate tectonics provides a link between the geological processes of sea-floorspreading, subduction, and mountain building. However, details of how this theory relates to ancientcontinental tectonic processes are unclear in many parts of the world. Although early enthusiasm ledmany geologists to propose paleogeodynamic reconstructions for ancient orogenic belts, many of thesemodels need revision in the light of increased understanding of accretion-related orogenic tectonicdeformation and postaccretion dispersion (Howell, 1989).

As in many other ancient continental areas, the tectonostratigraphic units that comprise the ArabianShield are mostly fault-bounded, and the geologic histories of neighboring units commonly havecontrasting elements. These fault-bounded ‘tectonostratigraphic terranes’ are allochthonous and werebrought together by processes of accretion. Because of the low regional metamorphic grade and the

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Figure 1: Simplified geologic map of the Arabian Shield showing the distribution of the majorophiolitic suture zones (ultramafic rocks), arc formations (pre-Murdama batholiths and ancientvolcanosedimentary rocks), gneiss domes, molasse basins (Murdama), syn- to postorogenic intrusions,postorogenic volcanosedimentary basins (Shammar and Jibalah groups), and major fault zonesattributed to the collisional phase of Shield evolution. Inset map shows Najd fault trend.

excellent exposures that result from a general absence of surficial cover, the Arabian Shield in SaudiArabia is a unique laboratory in which to study the mechanisms of continental accretion.

The Arabian Shield (Figure 1) has been mapped by the Saudi Arabian Deputy Ministry for MineralResources (now the Saudi Geological Survey) and its associates––the French Bureau de RecherchesGéologiques et Minières (BRGM) and the US Geological Survey (USGS). Since the early 1980s, the

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Shield has been regarded as the product of accretion in Neoproterozoic times of volcanic arcs (Fleck etal., 1980; Gass, 1981; Kröner, 1985; Stoeser and Camp, 1985). The evidence for such a history is theassociation of voluminous calc-alkaline volcanic and plutonic rocks separated by ophiolite-lined suturesthat divide the Shield into distinct tectonostratigraphic terranes (Johnson and Vranas, 1984; Stoeserand Camp, 1985).

A BRGM research project, involving the synthesis and re-evaluation of all available geologic andmetallogenic data on the Arabian Shield, led to a reappraisal of the structure and geologic history ofthe Arabian Shield. The project involved the following: (1) new field and geochemical andgeochronological work in several areas of the Shield; (2) the compilation of 1:250,000-scale geologicmaps; (3) the production of a new aeromagnetic map of the Shield; and (4) an exhaustive synthesisand re-evaluation of geochemical, geochronologic, metallogenic, and mineral-exploration data.

Some of the main geologic results of this work are presented here. The synthesis and re-evaluation ofthe metallogenic data will be published in a separate paper elsewhere. All data will be available as adigital report and attached databases (ArcView GIS) available from BRGM, France and the SaudiGeological Survey.

REGIONAL GEOLOGIC SETTING

Although Paleoproterozoic rocks may occur in the eastern part of the Shield, the formation of theShield essentially took place in the Neoproterozoic over a relatively short time span from about 900 to550 Ma (Figure 2). This led to the formation of a continental crust more than 40 km thick. Themorphology and present shape of the Shield are mainly the result of relatively recent geologic eventslinked to the opening of the Red Sea.

Before the Red Sea began to open at about 25 to 30 Ma (Camp and Roobol, 1992), the Arabian Shield of650,000 sq km formed part of a larger geologic unit known as the Arabian-Nubian Shield. Today, the

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Figure 2: Chronology of geologic events inthe Arabian Shield and the distribution ofage determinations and their analyticalconfidence. Of the 507 available ages, 97 areconsidered to be of very good quality and anadditional 52 are acceptable with restrictions.Note the change in the time scale. MSWD =Mean Squares of Weighted Deviation.

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rocks of the Nubian Shield are exposed in eastern Egypt, Eritrea, western Ethiopia, northern Somalia,and in the Sudan, whereas the Arabian Shield occupies much of western Saudi Arabia and has smallerexposures in southern Levant, southern Jordan, and in Yemen. The exposed area of the Arabian Shieldis 2,200 km long from north to south and has a maximum width of about 700 km.

The Arabian-Nubian Shield consisted mostly of juvenile crust interpreted as an area of transpressivesuturing between East and West Gondwana (Johnson and Kattan, 2001). It formed through the accretionof mainly interoceanic island arcs along sutures marked by ophiolites (Bakor et al., 1976; Gass, 1981;Bentor, 1985; Kröner, 1985; Stoeser and Camp, 1985; Vail, 1985; Pallister et al., 1987; Quick, 1991; Al-Saleh et al., 1998; Johnson, 1998). This occurred between about 900 and 550 Ma as the MozambiqueOcean closed (Stern, 1994). It may also have included an oceanic plateau formed by the head of anupwelling mantle plume (Stein and Goldstein, 1996).

The accretion resulted in the formation of tectonostratigraphic terranes that are separated either bymajor suture zones––trending mainly north and northeast and lined by serpentinized ultramafic rocks(ophiolites and tectonic slices)––or by major NW-trending faults. Stoeser and Camp (1985), Cole (1988),and Johnson (1998) compiled tentative maps of the tectonostratigraphic terrane boundaries and theirassociated orogenic zones for the Arabian Shield (Figure 3).

The Arabian Shield of Saudi Arabia is only slightly metamorphosed (except for some areas of gneissicrocks) and it constitutes one of the best-preserved and well-exposed Neoproterozoic assemblagesresulting from the accretion of volcanic arcs. It is overlain to the east, north, and south by a thicksuccession of Phanerozoic sedimentary rocks, and is bounded to the west by the Red Sea that nowseparates the individual Arabian and Nubian shields.

Geochronologic Constraints

Over 500 U-Pb, Rb-Sr, Sm-Nd, K-Ar, and Ar-Ar age determinations of Arabian Shield rocks are availableeither as published data (see synthesis by Johnson et al., 1997) or obtained as part of this project. Theage determinations were re-evaluated analytically using the Isoplot software (Ludwig, 1991) andsubdivided into four confidence levels (Figure 2). They span a time interval from about 2,340 to 340Ma with three main maxima at about 730 to 720 Ma, 640 to 630 Ma, and 590 to 580 Ma. Acceptable agesfall in the range of 850 to 560 Ma with a maximum at around 630 to 620 Ma, and ages acceptable withminor restrictions are within the range of 900 to 550 Ma. Two geologically suspect Rb-Sr ages are fromWadi al Faqqh [1165 ± 110 Ma; Fleck et al. (1980)] and from the Khamis Mushayt batholith [495 ± 3 Ma;Qari (1985)].

AEROMAGNETIC DATA AND CONSTRAINTS

Several aeromagnetic surveys were carried out from 1962 to 1983 by commercial companies under theauspices of the Ministry of Petroleum and Mineral Resources of the Kingdom of Saudi Arabia, andsupervised by BRGM and USGS. The surveys were conducted over individual blocks. They hadground clearances of 150, 300, or 500 m and a line spacing of about 800 m. The 1962 and 1965 to 1967surveys supervised by BRGM covered the entire Shield, and were flown using fluxgate Gulf Mark IIImagnetometers with analog recording. In addition, five general and several less-extensive surveys,were carried out over targets of economic interest using a CSF cesium-vapor magnetometer in 1976and 1981, and a Geometrics G813 proton-precession magnetometer in 1983, both with digital recording.

The 1962 and 1965 to 1967 data were originally presented as total-intensity contour maps compiled at20-gamma intervals to a scale of 1:50,000. These data were later recompiled into a set of colored1:500,000-scale, 100-gamma-interval total-intensity maps and photographically compiled and reducedto 1:1,000,000 scale by Blank and Andreasen (1991). Subsequently, Georgel et al. (1985) digitized theanalytical data from the original records. Although more arduous than gridding the contour maps,this method allowed integration of all the original data without the filtering effect of the contouring.To produce the aeromagnetic map presented as Figure 4, we used analytical data that were digitizedfrom the original records rather than from the contoured maps.

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Because the original data obtained by digitizing the profiles do not correspond to the total magneticfield, a regional field was calculated for each profile and subtracted from the original data. The resultantdata were compiled to produce a map with an altitude of 300 m and a grid spacing of 500 m in aUTM-38 projection. No artifact appears between the different blocks, implying that the regional fieldwas correctly calculated. Full information about the original data can be provided upon request.

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Figure 3: Simplified geologic sketch map of the Arabian Shield showing the terranes and theirboundaries, and the main Pan-African structural features and sedimentary basins. Major faultzones, such as Ruwah, Ar Rikah, Halaban, and Qazaz, belong to the Najd fault system.

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Analysis of the Aeromagnetic Data

The aeromagnetic anomalies (Figure 4) generally correlate well with known lithostratigraphic units atlarge scales. Mafic and ultramafic rocks typically show much stronger anomalies than metasedimentaryrocks, whereas metavolcanics cause strongly disturbed patterns. Most of the aeromagnetic anomaliesare ‘normal’, whereas ‘reversed’ anomalies are mainly associated with Red Sea dikes and ultramafic-decorated fault zones.

In order to clarify the interpretation of the aeromagnetic map, we subdivided it into several sectorsbounded by some of the dominant aeromagnetic trends and lineaments. These boundaries, eithersharp or gradual, generally corresponded to the province boundaries of Delfour (1979), and to theterrane boundaries defined by Johnson and Vranas (1984), Stoeser and Camp (1985), and Johnson(1998), for which we used the generally accepted nomenclature (Figure 3).

Major long-wavelength magnetic anomalies are observable on the 10-km upward continuationaeromagnetic map (not shown here but available at http://gisarabia.brgm.fr/geophysi.htm). In orderof spatial importance, the anomalies are associated with (1) Murdama-age basins in the northeast andnorthwest, (2) NW-trending Najd faults, and (3) E-trending anomalies in the south. The N-trendingNabitah orogenic belt and most of the terrane boundaries mapped by Johnson and Vranas (1984) andStoeser and Camp (1985) do not show up clearly on this map. This suggests that, unlike the Najd faultsystem, they are not major vertical fault zones but rather a series of thrust faults that do not extendvery deeply into the crust, as had been shown earlier by Gettings et al. (1986).

The southern terranesIn the south of the Shield, the NW-trending Ruwah fault zone forms the northern boundary of the Asirterrane. The fault zone is the southernmost element of the Najd fault system. Although geologicmapping and aerial and satellite imagery show that the Asir terrane is marked by mainly N-trendinglithostructural features, the predominant magnetic features in this region have a W- to NW-trend.

A very weak N-trending, long-wavelength discontinuity on the vertical-gradient map correlates wellwith the major northerly trending tectonic belts of Wadi Bidah and Wadi Shwas. The Nabitah suturezone in the easternmost part of the terrane is marked by a weak discontinuity in the dense E-W trendthat disappears eastward beneath the Phanerozoic sedimentary cover. This weak discontinuity issurprising, because the Nabitah fault zone is generally interpreted as a major fault zone lined byultramafic tectonic slices (Stoeser and Camp, 1985). Similarly, the transition to the Jiddah terranedefined by Johnson and Vranas (1984) and by Johnson (1998) is not marked by a clear-cut lineamentbut rather by a long-wavelength transition. The northern part of the Asir terrane and all of the Jiddahterrane show a large arcuate magnetic pattern open to the southwest.

The northern terranesThe Ad Dawadimi, Afif, Hijaz, and Midyan terranes are characterized by major long-wavelength,low-relief anomalies associated with the main Murdama, Thalbah, Hadiyah, and Furayh molasse basins(Figure 3) in the northeast and northwest, and with the Abt formation west of the Al Amar fault. Thelargest of them, the Murdama basin, appears as a 200-km-long elliptical, long-wavelength magneticanomaly that contains several local anomalies associated with posttectonic intrusions.

The Ad Dawadimi terrane (Delfour, 1979; Johnson and Vranas, 1984) is in sharp contrast to the easternAr Rayn terrane. It has very low magnetic relief and shows no clear trend. The few sharp magneticanomalies are associated with N-trending ultramafic tectonic lenses within the terrane and to subcircularposttectonic intrusions. The western border of the terrane is marked by a sharp, short-wavelengthmagnetic anomaly linked to the Halaban ophiolites. The intrusive complexes of the Nabitah faultzone, such as the Furayhah and Al Bara batholiths, have a typical sigmoidal fabric. Major positivemagnetic anomalies correlate with large gneiss domes associated with the Najd faults, such as theWajiyah and Hamadat domes in the north, and the Halaban and Jabal Kirsh domes in the centralShield.

The Tharwah-Bi’r Umq suture zone at the limit between the Jiddah and Hijaz terranes is only weaklymarked and is parallel to the arcuate fabric noted in the northern part of the Asir and Jiddah terranes.

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Similarly, the northern Yanbu suture zone between the Midyan and Hijaz terranes is only marked byshort-wavelength anomalies associated with ultramafics, and is offset by the Najd faults. Close to theRed Sea, the main NW-striking Najd pattern is spatially associated with negative magnetic lineamentsthat are subparallel to the coast and mark major Cenozoic dikes.

The Ar Rayn terrane (Delfour, 1979; Stoeser and Camp, 1985) shows a mosaic of anomalies with noclear trend. Ultramafic rocks of ophiolitic affinity line the Al Amar fault on the western boundary ofthe terrane. Johnson and Stewart (1995) showed that the magnetic anomaly of the Ar Rayn terraneextends at least as far north as latitude 29°N and probably to the Zagros suture, and that the Al Amarfault––although largely obscured by Paleozoic deposits––is a major regional structure.

Figure 4: Aeromagnetic map of the Arabian Shield as a raster image of the magnetic field reducedto the pole.

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A mosaic of short-wavelength anomalies characterizes the northern Ha’il terrane. Their marked N toNW orientations reflect the Nabitah trend and Najd trend, respectively. The southeastern limit showsa progressive transition to the Murdama basin. An anastomosing sigmoidal network of dominantlyNajd faults and Nabitah structures poorly defines the southwestern limit of the Ha’il terrane.

Well-defined subcircular and annular anomalies scattered throughout the Shield are the signatures ofmajor posttectonic intrusions and ring complexes that range in composition from gabbro to alkaline,peralkaline, or peraluminous granites and syenites.

Major Lineaments

The aeromagnetic lineaments can be grouped into the following four main trends (Figures 4 and 5):

1. A dominant Najd trend of N135ºE to N150°E, clearly related to the Najd faults and presentthroughout most of the Arabian Shield;

2. A Red Sea trend of N150ºE to N160°E, parallel to the coast and found only in the hinterland;3. A minor N-S trend associated with the Nabitah fault zone, which appears faintly on the aeromagnetic

map, but is largely obscured by the Najd lineaments;4. An E-W trend that is widespread in the southern Asir terrane.

ARABIAN PLATE ACCRETION AND CRATONIZATION

Precratonic Formations and Events (before 680 Ma)

Old continental crust––Where is the eastern margin of the Arabian Shield?The Arabian-Nubian Shield formed through the accretion of several volcanic arcs along sutures markedby ophiolites (Bakor et al., 1976; Gass, 1981; Bentor, 1985; Kröner, 1985; Stoeser and Camp, 1985; Vail,1985; Pallister et al., 1987; Quick, 1991; Johnson, 1998; Al-Saleh et al., 1998), between about 900 and550Ma. Accretion took place as East and West Gondwana collided and the Mozambique Ocean closed(Stern, 1994).

As a result, the Arabian-Nubian Shield is not generally underlain by old continental crust. However,terranes with pre-Pan-African continental affinities have been identified on the basis of zircon Pb-Pband U-Pb geochronology and radiogenic-tracer studies. They are found along the eastern margin ofthe Shield in Saudi Arabia and Yemen, and west of the River Nile (Stacey and Hedge, 1984; Stacey andAgar, 1985; Stoeser and Camp, 1985; Stoeser and Stacey, 1988; Schandelmeier et al., 1990; Agar et al.,1992; Windley et al., 1996, and references therein).

In Saudi Arabia, 12 age determinations of between 2,340 and 1,000 Ma (see Figure 2) were publishedby Aldrich et al. (1978), Fleck et al. (1980), Calvez et al. (1983, 1984), Calvez and Delfour (1985), Staceyand Hedge (1984), Stacey and Agar (1985), Stoeser and Stacey (1988), and Agar et al. (1992). Thesamples are all from the Al Amar fault zone and the area to the south within the Asir terrane. Mostdata sets have a Mean Squares of Weighted Deviation too high to be acceptable on analytical grounds;others are clearly mixed-zircon grains. The only analytically acceptable old date is a Rb/Sr date of1,165 ± 110 Ma (Fleck et al., 1980) from Wadi al Faqh in the Asir terrane. However, it is anomalouslyold in comparison with nearby zircon data that provided an age of 842 ± 17 Ma (Kröner et al., 1992).

The old zircon ages are probably from zircon grains that were either eroded from ancient continentalcrust or incorporated into plutonic rocks as xenocrysts. However, evidence for old continental crust isalso provided by lead-isotope data from galena and feldspars (Stacey et al., 1980; Stacey and Stoeser,1984; Bokhari and Kramers, 1982; Stacey and Agar, 1985). They show that the western and southernparts of the Shield have oceanic affinities, whereas the eastern part of the Shield has a more continentalsignature. On this basis, the southern portion of the Afif terrane was considered by Stoeser and Camp(1985) to be underlain, at least in part, by continental basement of Paleoproterozoic to Archean age.

Similarly, old ages around 1,200 Ma have been reported by Pallister et al. (1987) for xenocrystic zirconswithin gabbro of the Thurwah ophiolite. The xenocrysts were interpreted as inclusions of detritalzircon from metasedimentary rocks in the structural basement.

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Previous models for the formation of the Arabian Shield favored repeated rifting of an Archean toMesoproterozoic craton, involving the formation and closure of small ocean basins (Delfour, 1979;Kemp et al., 1980; Delfour, 1979). Field and geochronological data indicating the absence of oldcontinental crust in the western part of the Shield, imply that these models are no longer tenable. Thisis further supported by the ultramafic and associated rocks within the suture zones being slightlyolder (or of the same age) as the dated rocks on either side.

Within the Precambrian of Yemen, Windley et al. (1996) identified four gneissic and two island-arcterranes based on mapping, and on geochronologic and isotopic data. The two western gneiss terranes

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are correlated with the Afif and Asir terranes in Saudi Arabia. To the east of these, the Abas and AlMahfid gneiss terranes yielded Nd model ages of about 2,700 to 1,300 Ma. They could not be correlatedwith any documented terranes in Saudi Arabia.

In conclusion, there is clear isotopic and geochronological evidence for Paleoproterozoic and Archeancrust in the eastern part of the Arabian Shield (Whitehouse et al., 2001). However, the absence of cleargeologic indicators and of a marked aeromagnetic anomaly, leads us to consider that the easterncontinental margin of the Arabian Shield has to be looked for beneath the thick Phanerozoic sedimentarysuccession of the Arabian Platform.

Ophiolitic suture zones: remnants of precratonic oceanic lithosphere?The presence of ophiolites and their association with calc-alkaline volcanic rocks of island-arc affinity,suggests that the continental crust of the Arabian Shield developed by a process of horizontal crustalaccretion of oceanic terranes between about 900 to 680 Ma (Bakor et al., 1976; Greenwood et al., 1980;Gass, 1981; Kröner, 1985; Vail, 1985; Stoeser and Camp, 1985; Quick, 1991; Johnson, 1998). This impliesthat several hundreds to even thousands of kilometers of oceanic lithosphere must have been subductedto produce the accreted crustal material that is incorporated into the approximately 40-km-thick crustof the Arabian Shield (Mooney et al., 1985; Stern, 1994).

Fragments of Neoproterozoic oceanic crust and mantle, for example, the Jabal Ess ophiolite (Shantiand Roobol, 1979; Pallister et al., 1987, and references therein), occur within several major fault zonesand are well-defined on the aeromagnetic map (Figure 4). Most available ages on the ophiolites wereobtained by U-Pb zircon dating of hornblende gabbro, quartz diorite, plagiogranite, and quartzkeratophyre from all the main ophiolitic suites of the Arabian Shield (Pallister et al., 1987). Pallister etal. (1987) applied a Pb-loss model to the data based on the assumption that lead-loss due to dilatancyin the zircons was caused by uplift during Tertiary Red Sea rifting (Cooper et al., 1979; Stoeser et al.,1984). Additional data on the ophiolites are provided by Sm-Nd isochron ages (Claeson et al., 1984;Dunlop et al., 1986) and Ar/Ar dating by Al Saleh et al. (1998). Pallister et al. (1987) showed that, withthe exception of the Jabal Ess ophiolite in the northeast of the Shield that predates the oldest rocks byas much as 40 Ma, all ophiolites are coeval with or slightly predate the early magmatic phases ofadjacent arcs. They interpreted this to mean that most ophiolites formed within the arc complexes,chiefly by back-arc spreading.

The consensus is that most, if not all, of the ophiolites are crustal remnants from small, short-livedocean basins that formed offshore in close proximity to the locus of tectonic accretion, rather thanbeing fragments of standard oceanic crust. Several geochemical studies have been published on themain ophiolite complexes; for example, the Turwah ophiolite in the Bi’r Umq suture zone (Nassief etal., 1984); the Darb Zubaydah ophiolite in the Nabitah suture zone (Quick, 1990); and the Jabal alWask (Bakor et al., 1976) and Jabal Ess (Shanti and Roobol, 1979) ophiolites in the Yanbu suture zone.Geochemical analyses indicate that the basalts of these ophiolites are not typical Mid Ocean RidgeBasalts in character. New Inductively Coupled Plasma Mass Spectrometry (ICP-MS) geochemicaldata (this project) from the pillow basalts of the Turwah ophiolite showed that they all have trace-element characteristics of lavas associated with present-day back-arc settings. Furthermore, thestratigraphic relations of several ophiolites indicated that they are commonly only a little older thanthe surrounding arcs (Shanti and Roobol, 1979). This, in turn, implied that the ophiolites must haveformed near to their accretion location, such as a marginal basin, rather than in a remote mid-oceanspreading-ridge setting. The common association of ophiolites with felsic lava, graywacke, turbidites,and the like, further supports this assumption.

The main ophiolite bodies were dated by several teams at around 838 to 828 Ma (Bi’r Umq), 782 to 706Ma (Yanbu), 698 Ma (Al Amar), and 680 Ma (Halaban) (Al-Saleh et al., 1998; Pallister et al., 1987;Quick, 1990). However, due to analytical uncertainties the formation age of the Nabitah suture-zoneophiolites is poorly constrained; according to Pallister et al (1987) it is about 751 to 709 Ma in the AdDafinah region but much older (847–823 Ma) farther north in the Bi’r Tuluhah region.

Present-day lithospheric plates move at velocities averaging 5 cm/year. With ophiolitic rocks withinthe Arabian Shield spanning a period of about 150 Ma, crustal displacements of several thousands of

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kilometers can be part of this history, involving rifting, amalgamation, accretion, rotation, anddispersion. This must be kept in mind when trying to reconstruct the paleotectonic settings of thevarious suspect terranes.

Volcanic arcsOur objective in this section is not to review the individual lithostratigraphic characteristics of thedifferent terranes. These have been discussed in detail in the Explanatory Notes to the various 1:250,000-scale geologic maps of the Arabian Shield and reviewed by several authors; for example, Johnson andVranas (1984); Stoeser and Camp (1985); Cole (1993); Johnson (1998). Instead, we outline the commonnature and specific characteristics of the terranes.

The number of suspected terranes (10 for Johnson and Vranas, 1984; 5 for Stoeser and Camp, 1985; 17for Cole, 1993; 8 for Johnson, 1998) remains a matter of geologic debate. Magnetic-data autocorrelationstudies (Galdeano, personal communication, this project) and magnetic-trend orientations show thatthe terranes cannot be distinguished on the basis of magnetic anisotropy (Figure 5). Similarly, anorientation study of Shield-wide dikes cannot be used to discriminate between different terranes (Figure6). No marked differences exist between them and only the southern part of the Shield (the Asir andJiddah terranes) has a slightly different orientation from the well-marked modal orientation of N100ºE.

The tectonostratigraphic terranes in the Arabian Shield are compositionally very similar, althoughtheir ages differ significantly. They generally consist of alternating belts of greenschist-grade volcanicand sedimentary rocks, and high-grade gneisses. Several geochemical studies of the volcanic belts inthe Arabian Shield have led to the recognition of their island-arc affinities (Greenwood et al., 1976;Roobol et al., 1983; Reischmann et al., 1983). The ICP-MS data obtained as part of this project confirmthese results. Similarly, the older-than-700 Ma tonalite-trondhjemite plutonic rocks have a trace-elementsignature typical of arc-related intrusive rocks. They include Al-depleted granitoid rocks, characteristicof present-day oceanic arcs, and adakitic-type plutons typical of present-day young, hot lithosphere-subducting plates.

Volcanic-arcs are generally sub-parallel to the structures that generate them and, when amalgamatedand accreted to a continental margin, remain sub-parallel to the suture zones unless disrupted bydispersive tectonics. The major volcanic belts that correspond to old volcanic arcs in the ArabianShield trend northward in the southern Asir terrane, northeast in the Jiddah and Hijaz terranes, andnorth in the eastern Afif and Ar Rayn terranes (Johnson and Vranas, 1984; Stoeser and Camp, 1985;Cole, 1993).

The arc-related aeromagnetic signatures are now largely obscured by a dominant and penetrativeNW-oriented Najd trend in the north that reflects the importance of transposition of pre-existing rocksalong the Najd fault system (Figure 4), and by an E-W trend in the south that signals major late orogenicintrusions.

Arc magmatism began in the southwestern Asir terrane. There the oldest dated rock is the Bidahtonalite (Rb-Sr, 901 ± 37 Ma; Marzouki et al., 1982), and the oldest dated volcanic rock is a basalt fromthe Kulada suite (Rb-Sr, 847 ± 34 Ma; Kröner et al., 1983). Toward the east and north, the terranesbecome progressively younger. The youngest terrane is Ar Rayn that is separated from the AdDawadimi terrane by the Al Amar fault zone along its eastern boundary.

The Ad Dawadimi terrane is composed of mafic to ultramafic ophiolitic complexes and clastic rocks.The ophiolite complexes consist of gabbro, dolerite, and plagiogranite (with chilled margins in places),and volcanic rocks, in a serpentinite matrix (Shanti and Gass, 1983). Clastic rocks of the Abt formationform a monotonous, 40-km-wide, tightly folded succession of sericite and chlorite schist (Delfour,1979). Detrital zircons from the Abt schist were derived from source rocks with a mean age of 710 Ma,and ophiolitic gabbro near Halaban yielded ages of about 690 to 680 Ma (Stacey et al., 1984; Al Saleh etal., 1998). These dates imply that the Abt schist was formed during this time period and that it predatesthe final closure of the oceanic basin that lay west of the Ar Rayn terrane. The 641 ± 25 Ma age ofsyntectonic leucogranite intrusive into the Abt schist gave a lower age limit for the schist (Stacey et al.,1984).

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286 774 128 284 868 218

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Figure 6: Rose diagrams of all dikes (N = 33,115) recorded on the 1:250,000-scale geologic maps ofthe Arabian Shield and plotted by 1° quadrangles. The Shield is crosscut by a dense network ofdikes that are well exposed and appear clearly on aerial and satellite imagery. They provide excellentmarkers of paleostress fields. The computed dikes have lengths of up to 30 km with a modallength slightly over 1 km. The rose diagrams take into account all measured dikes including aminor set of dikes, abundant along the Red Sea coast, that is associated with the opening of the RedSea. The number in each degree square is twice the total number of lineaments.

In contrast to the Ad Dawadimi terrane, the Ar Rayn terrane consists mainly of syntectonic tonalitic togranodioritic gneisses dated at less than 670 Ma (667 ± 17 Ma, Calvez et al., 1983; 645 ± 35 Ma, Baubronet al., 1976) that intrude typical arc-type calk-alkaline volcanic rocks. In turn they are intruded byyounger-than-645 Ma posttectonic monzogranite and granodiorite (Calvez et al., 1983; Stacey et al.,1984).

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Pan-African Cratonization (about 800 to 680 Ma)

The final stage of suturing of the various terranes is generally well constrained. It postdates theophiolites in the suture zones and the adjacent island-arc rocks, and predates the posttectonic stitchingintrusions and overlapping volcanic and sedimentary basins. The oldest suturing is that of Bi’r Umq,which can be dated at about 810 to 780 Ma, and the youngest is that of Al Amar of around 690 to 670Ma. This last suturing event caused a major orogeny and is interpreted as representing the finalclosure of the Mozambique Ocean basin that separated East Gondwana from West Gondwana.

Major fault zones marked by ultramafic rocks, such as serpentinite and listwaenite––as well as mylonite,phyllonite, or gneiss domes––now separate the tectonostratigraphic terranes. The fault zones form ananastomosing network of primarily left-lateral strike-slip faults. They include the inner part of theNabitah belt, several peripheral NW-trending Najd faults, and N-trending structures (Figure 3). Theinner 200-km-wide zone of the Nabitah belt is composed of large intrusions and sedimentary formationsthat underwent compressional deformation. Sigmoidal, fish-shaped shear zones are clearly visible onsatellite images, with the most striking features being the Furayhah batholith (Kemp et al., 1982) andthe Al Bara batholith (Letalenet, 1979). The outer zone, in contrast, is characterized by lateral slip,reflected by the presence of gneiss domes.

Most fault zones are characterized by subhorizontal lineations. However, a few N-trending faultsshow steeply dipping lineations concentrated in corridors as much as several kilometers wide. Theyare the Ad Dafinah fault (folds with vertical axes), the Al Amar fault (boudinaged tension gashes), andthe Nabitah fault (boudinaged quartz). Most subhorizontal lineations have generally left-lateralkinematic indications on NW-trending faults and show right-lateral movement along those faults thattrend northeast (Figure 3). Such indications are consistent with an ENE–WSW to E–W principal stressorientation (present-day configuration) during the final suturing of the Arabian Shield.

Early age of Najd faultsThe chronology of displacements along the Najd faults can be constrained by dating either the pre-existing rocks present as melange (such as ophiolite) within the faults, or the syntectonic gneiss domes,overlapping volcanic and sedimentary basins, and stitching plutons.

The syntectonic NW-trending Tin granodiorite gneiss in the Ruwah strand of the Najd fault zone(Figure 3) in the central part of the Shield provides the oldest indication of movement on the Najdfault system. The U/Pb zircon age of 683 ± 9 Ma (Stacey and Agar, 1985) for the Tin complex is veryclose to the 680 Ma Ar/Ar age obtained for the Halaban ophiolite by Al Saleh et al. (1998). It impliesvery early transpressive deformation synchronous with or immediately following the accretion of thelast terranes to the east. Younger ages on Najd faults are provided by deformed plutons that haveintruded the Asir terrane; for example, the Damar complex granodiorite has a U/Pb zircon age of 635± 5 Ma (Stoeser and Stacey, 1988).

Similarly, several NW-trending gneiss belts occur in the northeastern part of the Shield (Qazaz, Wajiyah,and Hamadat gneiss domes) that are in geographic continuity with the Ar Rikah left-lateraltranspressional Najd fault. Satellite imagery, field data, and aeromagnetic imagery have revealed aunique structural arrangement consisting of an anastomosing network of planar structures thatdemarcate large ‘fish-shaped’ units. Fieldwork has shown the existence of a network of ductile left-lateral transcurrent faults with a few right-lateral faults. The age of deformation is constrained byseveral isotopic ages. Some of the orthogneiss is derived from the Ar Ra’al granodiorite that wasdated at 636 ± 23 Ma (Rb/Sr; Calvez et al., 1982). Syntectonic tonalites emplaced in the Baladiyah andQazaz faults have been dated at 676 ± 4 and 672 ± 30 Ma (U/Pb on zircon and whole-rock Rb/Sr;Hedge, 1984). These three dates constrain the formation of the gneiss belts to between 680 to 630 Ma,that is similar to the age of the Ruwah fault zone of the Najd fault system.

The synchronous formation of the Najd faults and the Nabitah fault zone is also indicated by severaltrends on the aeromagnetic map that show very large curvatures and delineate major sigmoidal blocksbetween the Nabitah and Najd faults. This implies that the Nabitah belt cannot be used as a markerfor displacement on the Najd faults, as was done in earlier studies by, for example, Brown (1972)Davies (1982, 1984), Cole and Hedge (1986), and Johnson and Kattan (1999).

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Molasse basins in relation to the Najd faultsObvious relationships exist between gneiss domes and the adjacent sedimentary formations ofMurdama age. Sedimentologic and structural field studies of the basins during this project showedthat they were formed during intense crustal deformation, with the basins occupying synclinaldepressions and the domes representing the anticlines of the same crustal fold train. The kinematicprocesses were accompanied by deposition of sedimentary molasse successions in the basins, withlarge wedges and internal unconformities that sealed the successive movements of the basement.

The molasse basins (Figure 3) collected Murdama group and equivalent-age sediments derived fromthe progressive erosion of the Nabitah belt. They are the Murdama and related Maraghan, Bani Ghayy,and Arfan basins in the east of the Shield, the Ablah and Junaynah basins in the south, the Hadiyahand Thalbah basins in the northwest, and the Furayah and Ghamr basins in the central Shield. Thetotal sedimentary thickness is at least 10 km in places (this project). The minimum ages of the Maraghanand Murdama basins are provided by early-stage cross-cutting intrusions, such as the An Najadi daciticplug at 631 ± 12 Ma (Walker et al. 1994), the As Asfar granodiorite at 620 ± 7 Ma (Cole and Hedge,1986), the Darat al Jibu granodiorite at 614 ± 5 Ma (Cole and Hedge, 1986), and the Uwaiyah granodioriteat 611 ± 3 Ma (Agar et al., 1992).

The large Murdama basin to the east of the Nabitah belt, displays increasing degrees of deformationand metamorphism from east to west. It is bounded to the west by gneiss domes with foliation foldsthat have curved axes and merge progressively southwestward into the Nabitah structures andnortheastward into folds of the sedimentary basin. Deposition of the Murdama group occurred after677 Ma (age of the As Sawda tonalite; Stacey et al., 1984). This constrains its formation to a periodbetween 677 and 631 Ma that is synchronous with the development of the gneiss domes.

Other basins that formed at about the same time as the Murdama basin are the Bani Ghayy, Arfan,Furayah, Thalbah, Hadiyah, and Ablah. The Bani Ghayy basins form overlapping sedimentary andvolcanic assemblages in the central part of the Shield and are disrupted by the Najd faults. They aredated by an interlayered rhyolite that gave a zircon U-Pb age of 620 ± 5 Ma (Stacey and Agar, 1985).The central Furayah basin provided an analytically suspect Rb/Sr age of 633 ± 15 Ma on dacite fromthe Qidirah formation (Brown et al. 1978). In the southern part of the Shield, the Ablah basin has beendated at 642 ± 1 Ma (Genna et al., 1999) making it contemporaneous with the Murdama basins in thenorth of the Shield.

Early enthusiasm led geologists to propose paleogeodynamic reconstructions in which these molassebasins represented pre-accretion and synsubduction fore-arc and/or back-arc basins. For example,Quick (1991) suggested that the clastic sedimentary rocks of the Murdama group are relicts of a fore-arc basin synchronous with the Nabitah volcanic arc and Abt accretion wedge, all belonging to thesame subduction system. However, the chronologic constraints and geologic relationships presentedhere imply that they can no longer be viewed as such, but that they represent orogenic-foreland molassebasins related to the cratonization of the Arabian Shield.

Gneiss domes and molasse basinsAn important result from this study is an understanding of the genesis of the gneiss domes and theirclose relationship to associated intracontinental sedimentary basins. The gneiss domes fit into a left-lateral kinematic deformation pattern. They are contained within the Pan-African belt of the ArabianShield, whose axial zone is represented by the Nabitah belt, described above. Uplift of the high-gradegneisses occurred by transpression before the deposition of the Bani Ghayy group, that is, before 620Ma, and before or during the formation of the Murdama basins (670 to 630 Ma) prior to the emplacementof undeformed granites in the Shammar basin (630 Ma). The extent of vertical uplift can be estimatedto be about 10 to 15 km on the basis of the metamorphic assemblages of the Kirsh dome.

Late- and post-Pan-African Plutonic Activity

Abundant late and posttectonic plutons were intruded into the Arabian Shield between 680 and 550Ma (Figure 2). They have a great variety of compositions and shapes. The felsic intrusions are circular,

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elongate, or raindrop-shaped, whereas the mafic intrusions generally form sills. Dike swarms andring dikes give an indication of the existence and general shape of concealed batholiths.

Two main groups can be distinguished in this association; they are (1) a potassic calc-alkaline groupincluding mainly monzogranite, granodiorite, and alkali-feldspar granite (intruded between 680 and580 Ma); and (2) an alkaline group of alkali-granite, monzonite, and syenite (ranging in age from 635to 550 Ma).

This ‘postorogenic’ calk-alkaline magmatism distinguishes itself from subduction magmatism by itsgenerally felsic and highly potassic nature that suggests a crustal origin. Geochemical modeling studies(D. Thiéblemont, unpublished BRGM technical report, 1999) confirmed that partial melting of the arc-related tonalite-trondhjemite suite could produce melts with trace-element signatures similar to the‘postorogenic’ granitoids. This is in support of their origin by partial melting as a result of underplatingof the accreted magmatic arc terranes and general thickening of the Arabian Shield. However, thealkaline granites cannot be derived from a crustal tonalite-trondhjemite source and require theparticipation of intraplate mantle.

Late Pan-African Extension

The late-stage Neoproterozoic formations in Saudi Arabia indicate that crustal thickening broughtabout during the Pan-African orogeny was followed by an episode of crustal thinning associated withimportant volcanic activity, and a return to marine sedimentation.

Normal faults and late Najd faulting and associated basinsThe northern part of the Arabian Shield is cut by a system of late NW-striking faults (Figures 1 and 3)known as the Najd faults (Moore, 1979). They are left-lateral strike-slip faults that either follow or cutacross the margins of earlier Pan-African structures. They also control the formation of the Jibalahbasins that formed on relay-zones or bifurcations of the Najd faults and can be identified as pull-apartbasins.

Despite the fact that various authors mention extensional phases in the structural evolution of thebasement (for example, Camp, 1986; Bokhari and Forster, 1988), few normal faults have been describedin the Arabian Shield. However, our field observations led to the discovery of two major faults thathave principally normal slip––they are the Jabal Farasan and Wadi Fatima faults. The Jabal Farasanfault extends northeastward for about 200 km (Figure 7) as a corridor several hundred meters wide inwhich the pre-existing foliation has been refolded. The axial planes of these structures are horizontal.Shear planes that developed in a brittle environment dip about 30ºSE and display striations with anazimuth of 160º. The Wadi Fatima fault also has a northeasterly strike, and dips about 50ºNW. Dragfolds and tension gashes at right angles to the striations indicate normal slip.

The largest detachment fault attributable to identified gravitational sliding is located at the base of theMurdama basin in the northeastern Shield. Above this fault, various low-angle normal and reversefaults cut the Murdama molasse with a general north-northeasterly shear direction. These are primarilyfaults lined with quartz veins that were identified during mineral exploration in the Silsilah district ofthe Murdama basin (Récoché, Al-Jehani and Shanti, 1998; Récoché, Al-Jahdali, Khalil and Lopes, 1998).

Other signs of late extensionWidespread extension could have facilitated or induced the partial melting of the mantle that producedthe abundant postorogenic alkaline granite batholiths and volcanic rocks between 635 and 550 Ma.Major NW-trending short-wavelength anomalies are ubiquitous on the aeromagnetic map of the Shieldand crosscut the structural lineaments. They are oblique to the N-S Nabitah structures, sub-parallel tomajor dike swarms (Figure 6), and show up particularly well in the southern part of the Shield (Figure4). The NW-trending magnetic anomalies are mainly related to a major Shield-wide postorogenicintrusion event, expressed at the surface by major W- to NW-oriented dike swarms. The geometry ofthe dike swarms indicates N–S to NE–SW extension (Figure 6), which is surprising as the main orogenicNabitah belt has a northerly orientation. It implies that extension was not simply gravitational butwas accommodated by the NW-trending left lateral faults.

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Continued extension and erosion led to total peneplanation of the Shield and the deposition of thecontinental Siq and Yatib formations (late Early Cambrian) and the Saq Formation (Late Cambrian toEarly Ordovician).

Proposed model for the post-Nabitah extensionThree large structures (Figure 7) were preserved through the Paleozoic and may represent thecontinuation of the Jibalah basins (Delfour, 1970; Basahel et al., 1984) extensional event. They are theTabuk and Widyan basins in the north (having the Saq Sandstone at their base), and the Rub’ Al-Khalibasin to the south (containing the Paleozoic Wajid Sandstone and the Eocambrian salt formations)

Figure 7: Simplified structural framework of Neoproterozoic extension in the Arabian Shield.

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(Faqira and Al-Hauwaj, 1998). The ‘Jiddah basin’ may have been another unit to this assemblage,marked by the late Proterozoic to Early Cambrian carbonate-bearing Fatima formation (group) (Basahelet al., 1984).

Similar postorogenic extension has been documented in the Sinai Peninsula, where NW–SE extensionis marked by NE-trending dikes (Brooijmans, 1999; Blasband et al., 2000). Extension was associatedwith the intrusion of granite plutons and dikes (590–530 Ma), and high-temperature/low-pressuremetamorphism (Brooijmans, 1999) dated at 600 to 530 Ma. Al-Husseini (2000) suggested that the Najdfault system extends beneath the Paleozoic cover east of the Arabian Shield and accompanied NE-oriented intracontinental rifts in Oman, Pakistan, the Zagros Mountains, and the Arabian Gulf. Therelationship between this mainly E–W extensional phase and the extension that we document in theArabian Shield is still poorly understood and requires more studies.

CONCLUSIONS

Arc accretion and cratonization of the Arabian Shield produced gneiss domes and molasse basinsover a system of intracontinental shear zones. The resultant belt was then subjected to tangential late-orogenic extension and crustal thinning controlled by the Najd transcurrent faults. Widespread erosionbrought about gradual peneplanation of the Shield, and crustal thinning instigated a late-marinetransgression.

These conclusions have major implications concerning the respective meaning of the Nabitah andNajd fault zones. The NW-striking ‘Najd faults’ are generally viewed as late-stage, left-lateral strike-slip faults (Moore, 1979; Stoeser and Camp, 1985). Our work, however, shows that they are the resultof a two-phase evolution. Early left-lateral transpressional movements along the faults developedmajor gneiss domes and large volcanosedimentary Murdama-type basins. Later movements controlledthe deposition of clastic sediments in pull-apart basins.

The synchronous formation of the early Najd faults and the Nabitah fault zone implies that the Nabitahbelt cannot be used as a passive marker of the amount of cumulative displacement along the Najdfaults, as was done in earlier studies (Brown, 1972; Davies, 1982, 1984; Cole and Hedge, 1986; Johnsonand Kattan, 1999). However, late-stage, left-lateral brittle displacement led to the formation of theJibalah basins. Such displacement presumably also contributed to the opening of Najd-orthogonal,salt-filled rift basins in Oman, Pakistan, the Zagros Mountains, and the Arabian Gulf that form thefoundations of most of the hydrocarbon traps in the Arabian Platform, as demonstrated by Husseini(1988, 1989) and Al-Husseini (2000).

The Arabian-Nubian Shield was formed over a relatively short period, which led Reymer and Schubert(1986) to suggest an arc-addition rate of 310 cu km/km-arc-length/million years. This calculationassumes an area of 6 million sq km, a volume of 300 million cu km, a 300-my-interval, and an arclength of 2,500 km. This is one order of magnitude higher than the present-day arc addition rates of 30cu km/km-arc-length/my, and has led several authors to suggest that much higher spreading ratesexisted in the past than today (see Howell, 1989, for discussion). Although questioned by some authors,Stein and Goldstein (1996) used this high growth rate as evidence for a major oceanic-plateau componentto the Arabian-Nubian Shield. Not only has such an oceanic-plateau component not been demonstratedgeochemically in the Arabian Shield (this study), but the important dispersion/accumulation tectonicsexposed here imply that simple orthogonal convergent-type accretion models cannot be used to estimatethe continental growth rate of the Arabian-Nubian Shield.

ACKNOWLEDGMENTS

We thank the Saudi Geological Survey (SGS), as geological successor to the Saudi Arabian DeputyMinistry for Mineral Resources, for permission to publish this paper. This reappraisal of the geologyof the Arabian Shield would not have been possible without the determination and enthusiasm of DrM.A. Tawfiq (Director SGS) and Mr F. Le Lann (then Director of the BRGM Geological Mission toSaudi Arabia). We thank them for their help in providing data and logistical assistance.

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Our work was helped by members of the BRGM Geological Mission, in particular by Philippe Bernard,Jean-Michel Eberlé, and Jean-Marc Leistel. We thank Marinus Kluyver and Patrick Skipwith for theirtranslation assistance. We also thank GeoArabia’s editors and two anonymous reviewers who greatlyimproved the paper. The design and drafting of the final figures was by Gulf PetroLink.

REFERENCES

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Aldrich, L.T., G.F. Brown, C.E. Hedge and R. Marvin 1978. Geochronologic data for the Arabian Shield;Section 1, Radiometric age determinations of some rocks from the Arabian Shield; Section 2,Tabulation of Rb-Sr and K-Ar ages given by rocks of the Arabian Shield. US Geological SurveySaudi Arabian Project Report PR-240, 20 p.

Al-Husseini, M.I. 2000. Origin of the Arabian Plate structures: Amar collision and Najd rift. GeoArabia,v. 5, no. 4, p. 527–542.

Al-Saleh, A.M., A.P. Boyle and A.E. Mussett 1998. Metamorphism and Ar40/Ar39 dating of the Halabanophiolite and associated units: evidence for two-stage orogenesis in the eastern Arabian Shield.Journal of the Geological Society, London, v. 155, p. 165–175.

Bakor, A.R., I.G. Gass and C.R. Neary 1976. Jabal Al Wask, northwestern Saudi Arabia: an Eocambrianback-arc ophiolite. Earth and Planetary Science Letters, 30, p. 1–9.

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ABOUT THE AUTHORS

Pierre Nehlig is Manager of the National Digital Map Database project at theFrench Bureau de Recherches Géologiques et Minières (BRGM). He joined BRGMin 1990 after a postdoctoral fellowship at the University of Washington. For fiveyears he managed a research project on the Arabian-Nubian Shield. Pierre is aquantitative earth scientist with extensive research, commercial, and teachingexperience in geology and geochemical and geophysical analysis as applied tocontinental formations, ore deposits, and the sea floor. His scientific interests coverthe fields of hydrothermal systems and related mineralization, the evolution of oceaniclithosphere from accretion to subduction, and continental growth and evolution.His broad experience covers the USA, Saudi Arabia, Oman, India, Indonesia, Kenya,New Zealand, Europe, and the Pacific. Pierre is a guest lecturer at the universities of Paris, Orleans, andTours. He is the author of more than 50 peer-reviewed papers in international journals, and of over 100presentations at international and national meetings.

Corresponding author. E-mail: nehlig@brgm.fr

Antonin Genna is a Structural Geologist with the French Bureau de RecherchesGéologiques et Minières (BRGM). He joined BRGM in 1989, working first onreconnaissance studies to investigate potential radioactive-waste storage sites. Soonafter, he began to concentrate on mineral exploration and mapping in France andin Indonesia, Canada, Saudi Arabia, Morocco, Romania, Gabon, Bolivia, India,Russia, and elsewhere, mainly for gold, base metals, and industrial minerals. Hehas extensive field experience in basin analysis, mapping, and mineral exploration.Antonin has been involved in many mapping, mineral exploration, and researchprojects in the Arabian Shield since 1990 and has extensive knowledge of itsstructural evolution. His primary scientific interest is in the early deformationthat accompanied the formation of sedimentary basins.

E-mail: a.genna@brgm.fr

Fawzia Asfirane is a Research Scientist with the French Bureau de RecherchesGéologiques et Minières (BRGM). She obtained a PhD in Geophysics at the ÉcoleNormale Supérieure de Paris in 1994 on the processing and interpretation ofaeromagnetic data from northern Algeria. Fawzia then joined Total where she workedon seismic exploration data using wavelet transforms for controlling zero phasing.At BRGM she has been focusing on aeromagnetic data of French Brittany and ofthe Arabian Shield, using various processing techniques to interpret anomalies interms of their geological significance.

E-mail: f.asfirane@brgm.fr

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Manuscript Received April 10, 2001

Revised September 30, 2001

Accepted October 7, 2001